Diphasiastrum species have been assumed to produce homoploid hybrids whose reproductive competence is still a matter of debate. Using flow cytometry, Bennert et al. demonstrate that the three Central European primary hybrids are consistently homoploid. Their nuclear DNA amounts are invariable and intermediate between the parents; no indications for diploid backcrossing are found. Higher DNA amounts occur in three presumably triploid populations, which arose by a secondary hybridization event, probably involving unreduced diplospores formed by a diploid hybrid.